Image sensors are used in a variety of digital image capture systems, including products such as scanners, copiers, and digital cameras. The image sensor is typically composed of an array of light-sensitive pixel cells that are electrically responsive to incident light reflected from an object or scene whose image is to be captured.
For example, a CMOS imager includes a focal plane array of pixel cells, each cell includes a photosensor, for example, a photogate, photoconductor or a photodiode overlying a substrate for producing a photo-generated charge in a doped region of the substrate. In a CMOS imager, the active elements of a pixel cell, for example a four transistor (4T) pixel cell, perform the necessary functions of (1) photon to charge conversion; (2) resetting a floating diffusion region to a known state; (3) transfer of charge to the floating diffusion region; (4) selection of a pixel cell for readout; and (5) output and amplification of a signal representing a reset voltage and a pixel signal voltage based on the photo-converted charges. The charge at the floating diffusion region is converted to a pixel or reset output voltage by a source follower output transistor.
Exemplary CMOS imaging circuits, processing steps thereof, and detailed descriptions of the functions of various CMOS elements of an imaging circuit are described, for example, in U.S. Pat. Nos. 6,140,630, 6,376,868, 6,310,366, 6,326,652, 6,204,524 and 6,333,205, all assigned to Micron Technology, Inc. The disclosures of each of the forgoing patents are hereby incorporated by reference herein in their entirety.
FIG. 1 illustrates a block diagram of one exemplary CMOS imager 108 having a pixel array 140 comprising a plurality of pixel cells arranged in a predetermined number of columns and rows, with each pixel cell being constructed as illustrated and described above. Electrically connected to the array 140 is signal processing circuitry for controlling the pixel array 140, as described herein. The pixel cells of each row in array 140 are all turned on at the same time by a row select line (not shown), and the pixel cells of each column are selectively output by respective column select lines. The row lines are selectively activated by a row driver 145 in response to row address decoder 155. The column select lines are selectively activated by a column driver 160 in response to column address decoder 170. Thus, a row and column address is provided for each pixel cell.
The CMOS imager 108 is operated by a timing and control circuit 150, which controls address decoders 155, 170 for selecting the appropriate row and column lines for pixel readout. The control circuit 150 also controls the row and column driver circuitry 145, 160 such that they apply driving voltages to the drive transistors of the selected row and column lines. The pixel column signals, which typically include a pixel reset signal Vrst and a pixel image signal Vsig, are output to column driver 160, on output lines, and are read by a sample and hold circuit 161. Vrst represents a reset state of a pixel cell. Vsig represents the amount of charges generated by the photosensitive element of a pixel cell in response to applied light during an integration period. A differential signal (Vrst-Vsig) is produced by differential amplifier 162 for each readout pixel cell. The differential signal is digitized by an analog-to-digital converter 175 (ADC). The analog-to-digital converter 175 supplies the digitized pixel signals to an image processor 180, which forms and outputs a digital image.
Under certain image capture conditions, an excess of white light, known as flare, can cause a captured image to have a low contrast or appear washed out. Flare can appear in images captured by any image sensor or scanner. Common sources of flare are internal reflections in low-quality lenses, the presence of dust in an optical system, and foggy environmental conditions. Flare can affect some or all of the captured image. In many cases, images exhibiting flare do not contain any deep black tones anywhere in the image, which is undesirable.
Thus, there exists a need and desire for an improved imager system and post-capture processing technique which can mitigate against image flare.